Since the development of off-axis holography in the 1960's, volume holography has been identified as a promising candidate for high density data storage. Theoretically, up to 10.sup.14 bits of information can be stored in 1 cm.sup.3 of a volume holographic medium. In addition, holographic storage promises fast data transfer rates, estimated at over 1 Gb/s. For general information on holographic memory systems, see for example the articles by Heanue et al. in Science 265: 749-752 (1994), Hong et al. in Optical Engineering 34(8): 2193-2203 (1995), and Psaltis and Mok in Scientific American 273(5): 70-78 (1995), or U.S. Pat. No. 4,927,220 (Hesselink et al.) and U.S. Pat. No. 5,450,218 (Heanue et al.).
One of the major challenges facing holographic data storage has been increasing the capacity of storage systems. Several approaches have been used for multiplexing, or storage of multiple pages within a system. Typical approaches include spatial, angular and phase-code multiplexing. For an overview of these techniques, see for example the above-mentioned article by Hong et al.
Three major noise sources affect the performance of typical holographic storage systems. Imperfections in detectors cause detector noise. Imperfections in the medium structure cause undesired scatter, which is independent on the number of pages stored in the system. Interpage crosstalk leads to a reconstruction of undesired pages when the medium is accessed with a reference beam corresponding to a given page. The crosstalk-limited SNR (object signal intensity/crosstalk intensity) varies inversely with the number of stored pages, for a fixed average intensity per page. For a review of interpage crosstalk for angular and phase-code multiplexing see for example the article by Bashaw et al. in J. Opt. Soc. Am. B, 11: 1820-1836 (1994), herein incorporated by reference. Interpage crosstalk is an important barrier facing efforts to increase the capacity of holographic storage media.